In electrophotographic printing systems, an electrostatic latent image is formed on a photoconductor and developed into a visible image by bringing the photoconductor into close proximity or contact with charged toner particles (also referred to in the art as marking particles). In a two-component developer, the toner particles becomes tribocharged and are attracted to the electrostatic latent image regions of the photoconductor.
After the electrostatic latent image on the photoconductor has been developed, the developed image is generally transferred to a receiver medium, such as a sheet of paper or transparency stock. This is typically accomplished by applying an electric field in such a manner to urge the toner particles from the photoconductor to the receiver medium.
In some configurations, it is preferable to first transfer the developed image from the photoconductor to an intermediate transfer member, and then from the intermediate transfer member to the receiver medium. Again, this is commonly accomplished by applying an electric field to urge the developed image toward the intermediate transfer member for the first transfer, and toward the receiver medium for the second transfer.
For multi-color images, the process of forming an electrostatic latent image and developing the image typically occurs in a plurality of separate electrophotographic modules, one for each color. The developed color separations are then either accumulated onto an intermediate transfer member or directly onto the receiver medium with multiple transfer steps, one for each color separation.
After the toner image has been transferred to the receiver medium, the receiver medium bearing the transferred toner image is then passed through a fusing device to permanently affix the developed image to the receiver medium by heat and pressure.
In the earlier days of electrographic printing, the marking particles were relatively large (e.g., on the order of 10-15 μm). As a result the print image had a tendency to exhibit a relief appearance (variably raised surface). Under most circumstances, the relief appearance was considered an objectionable artifact in the print image. In order to improve image quality, and to reduce relief appearance, over the years, smaller marking particles (e.g., on the order of less than 8 μm) have been formulated and are more commonly used today. This has the additional advantage of reducing image granularity.
With the improved print image quality, print providers and customers alike have been looking at ways to expand the use of electrographically produced prints. In certain classes of printing, a tactile feel to the print is considered to be highly desirable. Specifically, ultra-high quality printing such as for stationary headers, business cards, or greeting cards and invitations, utilize raised letter printing to give a tactile feel to the resultant print output. In the offset printing industry, this is typically carried out via thermography in an offline process. Some other applications where providing tactile feel in the printed image would be desirable are documents including Braille or other features adapted to be sensed by a visually-impaired person, or documents including tactile security features.
U.S. Pat. No. 7,783,243 to Cahill, et al., entitled “Enhanced fuser offset latitude method,” and U.S. Pat. No. 8,358,957 to Tombs, et al., entitled “Selective printing of raised information by electrography,” both disclose the electrophotographic production of printed images including raised tactile features using a larger-sized (e.g., 12-30 μm) clear (e.g., non-pigmented) toner applied together with small-sized (<9 μm) pigmented toners. U.S. Pat. No. 8,626,015 to Aslam, et al., entitled “Large particle toner printer,” discloses a printer for printing images including raised tactile features using a smaller-sized (3 to 9 μm) toner applied together with a larger-sized (e.g., >20 μm) toner having a charge-to-mass ratio that is between ⅓ and ½ the charge-to-mass ratio of the smaller-sized toner. In all of these configurations, the smaller-sized pigmented toners are first to be deposited onto the receiver, and the larger-sized clear toner is last to be deposited onto the receiver and consequently fused atop the smaller-sized toner. This laydown order is advantageous for the electrostatic transfer process since it is difficult to efficiently transfer the smaller-sized toner on top of the larger-sized toner. For example, if the larger-sized clear toner was applied first, the maximum electric field achievable in the transfer nip of the downstream printing modules would be reduced to a much lower level due to the Paschen limit for the larger air gaps created by the larger-sized toner. However, it has been found that the resulting fused images suffer from color desaturation due to the thick layer of clear toner that is placed above the color toner. This limits the color gamut achievable for raised printing using these approaches.
U.S. Pat. No. 6,993,269 to Yamauchi, et al., entitled “Image forming apparatus, image processing apparatus, image forming method and image processing method for forming/processing a three-dimensional image,” discloses the electrophotographic production of raised prints having the tactile effect produced by using a foamable toner in contact with the substrate. The disclosed electrophotographic apparatus utilizes an intermediate transfer process where multiple toner images (both color and clear) are accumulated on an intermediate transfer member and subsequently transferred as an integral mass of toner onto the substrate. The color and clear foamable toner are of similar size prior to fusing, as shown in the figures and inferred by the description of the printed image only becoming three dimensional after fusing. This approach has the additional disadvantage that making a foamable toner adds another level of complexity and cost to the toner formulation.
U.S. Pat. No. 4,459,344 to Jacob, entitled “Method for producing raised images by xerographic means,” also discloses the use of foamable (i.e., thermally intumesced) toner to produce raised prints using an electrophotographic process. As stated above, the additional requirements of formulating a foamable toner are undesirable from a complexity and cost viewpoint.
There remains a need for a method to provide printed electrophotographic images having tactile features with excellent color saturation at a reasonable cost for both toner materials and machine hardware.